1. Field of the Invention
The present invention relates to a cardiovascular information measurement system capable of noninvasively and continuously measuring vital cardiovascular information.
2. Description of the Related Art
Various indices such as a volume flow of blood, viscoelasticity caused by vasodilation and vasoconstriction are conventionally used as cardiovascular indices for grasping the states of circulatory organs of a patient.
A volume flow of blood is measured by the following method. The blood vessel of a patient is irradiated with an ultrasonic wave, the blood flow velocity is obtained using a Doppler signal generated by the blood flow. The volume flow of blood is then obtained in accordance with the relationship between the blood flow velocity and the cross-sectional area of the blood vessel.
An optical flow method for measuring a velocity vector using the liminance gradient of continuous image data is used as a method of obtaining vasodilation and vasoconstriction rates.
When the volume flow of blood is to be obtained from the blood flow velocity calculated using the Doppler signal, demand has arisen for a technique for easily and continuously calculating the cross-sectional area of the blood vessel of interest in order to obtain the volume flow of blood.
The above optical flow method undesirably requires complicated sequential calculations.
A method of winding a manchette on an arm portion of a patient and measuring the blood pressure from the resultant vibration or sound has been practiced as a conventional method of measuring the blood pressure. However, when the blood pressure must be measured for several days upon surgical operation, the arm portion of the patient must be pressurized to result in considerable mental and physical loads.
To solve this problem, a method of measuring, with an ultrasonic wave, the propagation velocity of a pulse wave propagating through the blood vessel and measuring the blood pressure is disclosed in Japanese Patent Laid-Open No. 7-241288. According to this method, the blood pressure is calculated using the following theoretical expression: EQU .DELTA.P=.rho.C.DELTA.v
where
.DELTA.P: the pressure change PA1 .rho.: the blood density PA1 .DELTA.v: the flow velocity change PA1 C: the pulse wave velocity
This theoretical expression is disclosed in the following reference (edited by The Japan Hydraulics & Pnuematics Society, New Hydropneumatic Handbook, Section 1, Chapter 3, 24-25, OHM-sha, Ltd., 1988). However, this theoretical expression is an expression based on a condition that no change occurs in the blood vessel wall. That is, this expression is made in no consideration of the influence of the blood vessel wall.